Fundamental understanding of cellular processes in health and disease has been gained by studying cells of various tissues in vitro. However, results obtained from experiments in vitro are often not sufficient to explain the performance of cells in more physiological settings like whole organs or living organisms. To place observations made in an in vitro system into a physiological context, studies have to be performed under in vivo conditions. In recent years an increasing number of approaches have been made to investigate cell function in situ utilizing imaging techniques. Unfortunately, non-invasive imaging techniques like computer tomography (CT), magnet resonance imaging (MRI), positron emission tomography (PET) or bioluminescence imaging (BLI) lack cellular resolution1. On the other hand, confocal and two-photon laser-scanning microscopy (LSM) provide sub-cellular resolution but have a fairly limited working distance and imaging depth2. Accessing target cells for the application of LSM is mostly invasive and often excludes the possibility of repetitive examinations.